1. Field of the Invention
The present invention relates to a PLL frequency synthesizer.
2. Related Background Art
In general, a PLL frequency synthesizer includes a voltage-controlled oscillator, a phase comparison unit, a charge pump, and a loop filter, and outputs an oscillation signal having a frequency which is a constant multiple of a frequency of a reference oscillation signal.
The PLL frequency synthesizer operates as follows. A control voltage value is input to the voltage-controlled oscillator, and an oscillation signal having a frequency corresponding to the control voltage value is output from the voltage-controlled oscillator. The oscillation signal output from the voltage-controlled oscillator or a signal having a frequency obtained by dividing the frequency of the oscillation signal is input to the phase comparison unit as a feedback oscillation signal. Also, a reference oscillation signal is input to the phase comparison unit. The phase comparison unit detects a phase difference between the feedback oscillation signal and the reference oscillation signal. A phase difference signal indicating the detected phase difference is output. A charging/discharging current corresponding to the phase difference indicated by the phase difference signal is output from the charge pump receiving the phase difference signal. The charging/discharging current is input to the loop filter. A control voltage value output from the loop filter is input to the voltage-controlled oscillator. Thus, an oscillation signal having a frequency which is a constant multiple of a frequency of the reference oscillation signal is output from the PLL frequency synthesizer.
In general, the loop filter includes a resistor and a capacitive element. A first end of the resistor is connected to an output end of the charge pump, and is connected to an input end of the voltage-controlled oscillator. A second end of the resistor is connected to a reference potential via the capacitive element. Among time constants representing characteristics of the loop filter, a natural frequency ωn is expressed by the following Equation (1), and a damping factor ζ is expressed by the following Equation (2). Here, Ipmp [A] is a magnitude of the charging/discharging current output from the charge pump, Kvco [Hz/V] is a gain of the voltage-controlled oscillator, C [F] is a capacitance value of the capacitive element, and R [Ω] is a resistance value of the resistor.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                ω          n                =                                            Ipmp              ·              Kvco                        C                                              (        1        )                                          [                      Equation            ⁢                                                  ⁢            2                    ]                ⁢                                                                                      ζ        =                              R            2                    ⁢                                    Ipmp              ·              Kvco              ·              C                                                          (        2        )            
An example of the above-described related art is disclosed in Japanese Unexamined Patent Application Publication No. 2002-280898.